1. Technical Field
The present invention relates to power supplies in general, and in particular to switching regulators. Still more particularly, the present invention relates to an apparatus for reducing noise from a group of switching regulators.
2. Description of the Related Art
Switched mode power supplies are commonly employed in many of today""s electronic devices due to their high efficiency and good output power regulation. Generally speaking, a switched mode power supply converts a low-frequency (e.g., 50 or 60 Hz mains frequency), high-voltage alternating current (AC) to a high-frequency (e.g., 30 to 300 kHz) AC, using a switched mode power supply control circuit. Next, the high-frequency, high-voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated direct current (DC) output, which may be used to power an electronic device. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop.
A switched mode power supply may include an integrated circuit switching regulator having a power switch or transistor coupled in series with a primary winding of a transformer. Energy is transferred to a secondary winding of the transformer by turning the power transistor on and off in a manner controlled by the switching regulator to provide a clean and steady source of power at the DC output. A switching regulator samples a feedback current from the output of the DC output of the power supply. When the feedback current is below a regulation threshold, the power transistor is switched by the switching regulator at a constant frequency.
In many applications that contain sensitive circuits, it is important to minimize the noise from a switching regulator in order to maintain a relatively high signal-to-noise ratio. The switching regulator noise is sometimes referred to as the beat frequency noise, since it is the result of the sum and difference of individual switching regulator noise. One contributor to switching regulator noise is from the difference in switching frequencies among multiple switching regulators. For example, a first switching regulator operates at 100 kHz and a second switching regulator operates at 98 kHz will yield switching regulator noise at 2 kHz.
Prior art solutions for handling switching regulator noise include external linear post-regulators and synchronization of DC/DC converter clocks, but those solutions are not very satisfactory. Consequently, it is desirable to provide an apparatus for reducing switching regulator noise from multiple switching regulators.
In accordance with a preferred embodiment of the present invention, an apparatus for reducing switching regulator noise from outputs of multiple switching regulators includes an operational amplifier, an capacitor, a resistor and a pair of diodes. The switching regulators are connected in series to provide a positive output rail and a negative output rail. The output of the operational amplifier in connected to an inverting input of the operational amplifier and a positive sensing input of one of the switching regulators. The non-inverting input of the operation amplifier is connected to the positive output rail of one of the switching regulator via the capacitor. The resistor is connected between a positive voltage output of one of the switching regulators and the capacitors. The pair of diodes is connected between the positive voltage output of one of the switching regulators and the negative output rail of the switching regulators.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.